Generally, in High Voltage Direct Current (HVDC) systems, Alternating Current (AC) power produced in a power plant is converted into DC power and the DC power is transmitted, and a power receiving stage re-converts the DC power into AC power and supplies the AC power to a load. Such an HVDC system is advantageous in that power may be efficiently and economically transmitted via voltage boosting, and in that connections between heterogeneous systems and long-distance high-efficiency power transmission are possible.
A Modular Multilevel Converter (MMC) is connected to an HVDC system to realize power transmission and reactive power compensation. In the MMC, multiple sub-modules are connected in series with each other. In the MMC, sub-modules are very important components and are controlled by a separately provided controller. To use the high voltage from sub-modules as driving power for the sub-module controller, there is required a power supply apparatus, which converts the high voltage into the low voltage required for the sub-module controller.
FIG. 1 is an equivalent circuit diagram of an MMC, and FIG. 2 is a circuit diagram of a conventional power supply apparatus for a sub-module controller of an MMC. As is well-known in the art, the MMC is composed of one or more phase modules 1, and multiple sub-modules 10 are connected in series with each other in each phase module 1. Further, respective DC voltage terminals of each phase module 1 are connected to positive (+) and negative (−) DC voltage buses P and N. A high DC voltage is present between the DC voltage P and N buses. In each sub-module 10, two connection terminals X1 and X2 are formed.
A conventional power supply apparatus 20 for the sub-module controller of the MMC includes an energy storage unit 21 and multiple series-connected resistors 22, connected in parallel with the energy storage unit 21, wherein a DC/DC converter 23 is connected to some of the series-connected resistors.
When the power supply apparatus 20 for the sub-module controller is applied to an MMC that is connected to an HVDC system, a high voltage of several to several tens of kV, stored in the energy storage unit 23, must be converted into a low voltage of several to several tens of V required for the sub-module controller. For this, in the conventional technology, a resistor voltage-dividing scheme using multiple resistors 22 connected in parallel with the energy storage unit 21 is used. That is, the voltage charged in the energy storage unit 21 is divided into voltages including a voltage across some resistor R3, among the multiple series-connected resistors R1 to R3, and is thus converted into a low voltage.
However, the conventional power supply apparatus 20 is problematic in that unnecessary loss occurs due to the use of the resistors R1 to R3 for voltage division.